1. Field
The present invention pertains to subcarrier assignment for uplink/downlink communications using Orthogonal Frequency Division Multiple Access (OFDMA).
2. Description of the Related Art
Conventionally, in a subcarrier assignment scheme of OFDMA communication, contiguous base stations use different frequencies to avoid interference. Published Japanese Translation of PCT International Publication No. 2005-502218 and Published Japanese Translation of PCT International Publication No. 2004-529524 disclose a technique of subcarrier assignment for a base station where a subscriber (mobile station) detects interference and a subcarrier channel, and provides a base station with information that is used for subcarrier assignment at the base station. Japanese Patent Application Laid-Open Publication No. 2005-80286 discloses a technique in which, when an assignment of forward-direction channels is performed, transmission of control information and data that are exchanged between a mobile station and a base station is controlled respectively based on a cell pattern.
FIG. 8 is a diagram depicting subcarrier assignment according to the conventional art. As shown in FIG. 8, if the system consists of three cells, frequency bands are allocated to each cell (cell 1, 2, 3), respectively, in advance and subcarriers are assigned to users within the range of an allocated frequency band.
FIG. 9A is a diagram of a conventional carrier sensing scheme. In this example, the figure depicts two base stations (BS) BS1 and BS2, and three mobile stations (MS) MS1, MS2, and MS3, which execute carrier sensing for an uplink path.
MS1 and MS3 are in a cell 201 of BS1, and MS2 is in a cell 202 of BS2. MS1 uses frequency f1, and MS2 uses frequency f2. Circles in the figure indicate distribution of electric field intensity of signals. It is assumed here that a threshold for carrier sensing of MS3 is −100 dBm. MS3 is at a point where the intensity of signals from MS1 and MS2 is less than the threshold for carrier sensing of MS3. In this case, MS3 cannot sense signals of frequency f1 from MS1 or signals of frequency f2 from MS2, thereby concluding that both frequencies f1 and f2 can be used.
However, with the configuration shown in FIG. 8, even if traffic concentrates in certain cells, subcarriers that have been assigned to other cells cannot be transferred. As shown in FIG. 8, each base station uses a frequency band different from the other base stations in order to avoid interference. Consequently, one cell cannot change (increase) a ratio of the frequency band of the cell to a certain range of frequency. In the case of the configuration shown in FIG. 8, the reuse factor equals 3, and it has been difficult to make the reuse factor equal 1. If the reuse factor is reduced toward 1, frequencies can be assigned to cells flexibly and dynamically.
FIG. 9B is a diagram depicting interference after the carrier sensing shown in FIG. 9A. When MS3 tries to establish uplink communications (UL), MS3 executes carrier sensing, determines that frequencies f1 and f2 are both available, and uses frequency f1 or f2. If MS3 uses frequency f1, interference with frequency f1 of MS1 occurs at a region F1. If MS3 uses frequency f2, interference with frequency f2 of MS2 occurs at a region F2. Further, if MS3 moves around and the direction of the antenna of MS3 changes so that the regions F1 and F2 are located at BS1, BS1 is affected when receiving signals from mobile stations (for example, an error rate deteriorates), and the communication quality will deteriorate.